1. Field of the Invention
The present invention relates to an adaptive antenna apparatus that selects adaptive control processes according to a detected signal quality. In particular, the present invention relates to an adaptive antenna apparatus for receiving a radio signal with adaptive control using a plurality of antenna elements, which is provided to keep favorable communication quality, for example, in a portable radio communication apparatus of a mobile communication system or the like. The present invention also relates to a radio communication apparatus using the same adaptive antenna apparatus.
2. Description of the Related Art
An example of an apparatus for controlling an array antenna provided with an adaptive antenna apparatus according to a first prior art example is disclosed in Japanese Patent Laid-open Publication No. JP-2003-087051-A (referred to as a Patent Document 1 hereinafter). The apparatus for controlling the array antenna has the following configuration in order to perform adaptive control without giving the arrival angle of the received signal in advance, such that a main beam is directed to a desired wave, and nulls are directed to interference waves. The adaptive controller is provided to adaptively control an array antenna, which is an ESPER antenna including one radiating element and six parasitic elements. In this case, the adaptive controller executes adaptive control process based on the circular station of a spectrum according to a received signal y(n) upon receiving a learning sequence signal included in a radio signal transmitted from a destination transmitter by the radiating element of the array antenna, and a learning sequence signal d(n) which is identical to the transmitted learning sequence signal and generated by a learning sequence signal generator. Doing so, allows the adaptive controller to calculate and set reactance values xm of respective variable reactance elements for directing the main beam of the array antenna in the directions of the desired waves, and for directing the nulls in the directions of the interference waves.
Moreover, in an adaptive antenna apparatus according to a second prior art example disclosed in Japanese Patent Laid-open Publication JP-2004-064743-A (referred to as a Patent Document 2 hereinafter), an adaptive antenna apparatus arranged with a plurality of array branches includes the following components in order to achieve downsizing and lower electric power consumption:
A plurality of radiating antenna elements arranged so that spatial correlations therebetween become smaller, that is, at an interval “d” of d>>λ, where λ denotes a wavelength of a radio signal.
A plurality of parasitic antenna elements arranged so that spatial correlations to these respective radiating antenna elements become larger relatively, that is, at an interval “d” of d<λ/2.
Variable reactance elements that terminate these parasitic antenna elements.
A reactance controller for controlling the variable reactance elements based on received signals received by the radiating antenna elements.
A weighting controller that weights the received signals received by the plurality of antenna elements by means of respective weighting circuits, combines the weighted received signals by means of a combining circuit, and controls the weighting circuits such that a signal-to-interference and noise ratio S/(I+N) of a combined output signal becomes the maximum.
However, the adaptive antenna apparatus according to the first prior art example has the following problems. In the first prior art example, since the directivity is controlled by the one antenna element and the plurality of parasitic elements, it is necessary for the antenna element and the parasitic elements to be optimally arranged. Thus, the size and shape of the array antenna apparatus are limited. In the case of the first prior art example, the size of the array antenna apparatus is required to have a diameter of ½ wavelength and a height of ¼ wavelength. In other words, the wavelengths are 0.33 m and 0.15 m in the 900 MHz band and the 2 GHz band, respectively, which are radio frequency bands used by current portable telephones, and are longer than the size of each portable telephone. Therefore, the antenna size of the first prior art example is larger than the size of the portable telephone, and thus, the array antenna apparatus cannot be used as it is in the portable telephones. Also, there are such problems that the control over the directivity is limited and that the control algorithm becomes complicated.
Moreover, in the adaptive antenna apparatus according to the second prior art example, since the plurality of ESPER antennas are co-allocated and the reactance control and the weighing control for adaptive control are used in combination, there are more problems in that the size of the apparatus configuration becomes larger and in that the control process becomes complicated.